Molybdenum is often a significant by-product from copper/molybdenum concentrates produced from the flotation of copper porphyry ores. In some instances the economic success of a copper mining operation depends upon recovery of molybdenum (in the form of molybdenite, MoS.sub.2), as a byproduct from the concentrate.
Typically, copper porphyry ores contain molybdenite and one or more copper sulfide minerals, such as chalcopyrite (CuFeS.sub.2), chalcocite (Cu.sub.2 S) and other copper sulfides. These ores are usually treated by a flotation process, wherein the ore is ground to free the copper sulfides and molybdenite from the surrounding rock. A suspension of the ground ore is sent to a flotation cell, where gas, usually air, is dispersed into the suspension to form bubbles. Particles with hydrophobic surfaces adhere to the surfaces of the bubbles and are carried to the surface of the suspension as a froth. The surfaces of copper sulfide minerals and molybdenite are made more hydrophobic from the addition of flotation reagents, eg. collector and frother reagents. Hence, the froth formed on the top of the suspension is a concentrate containing copper sulfides and molybdenite, which is then separated from most gangue minerals and recovered as a bulk copper/moly concentrate. Collector flotation reagents used to enhance hydrophobic surfaces on the copper sulfide minerals are typically sulfhydryl compounds, such as xanthates, dixanthogens, dithiophosphates, thionocarbamates, and xanthate ethylformate.
To separate the molybdenite from the copper sulfides, the bulk copper/moly concentrate is treated to depress the copper sulfides, i.e. to selectively change the surface properties of the copper sulfides such that they become more hydrophilic. After treatment, the bulk concentrate is again subjected to a flotation process in order to produce a concentrate of molybdenite. In this instance, particles of copper sulfide minerals which are depressed are not carried to the surface by the bubbles, whereas molybdenite particles, which have essentially retained their hydrophobic surfaces, are carried into the froth phase on the top of the suspension by the air bubbles, and are, thereby, separated from the copper sulfide mineral particles. Depression of the copper sulfide minerals is usually accomplished by chemical treatment using, for example, alkali sulfide reagents, Nokes reagents, cyanides (including ferro- and ferri-cyanides), and chemical oxidants, sometimes combined with thermal treatments, such as roasting or steaming.
Chemical treatments involve conditioning the concentrate with alkali and alkali earth sulfides, Nokes reagents, and/or cyanides. Chemical treatments are believed to function mainly by displacing the collector molecules on the surface of the mineral particles to produce a hydrophilic state upon the surface. Examples of chemical treatments are described in U.S. Pat. Nos. 2,492,936 to Nokes et al., and 4,549,959 to Armstrong et al.
A disadvantage with chemical treatments is that the collector used during the initial flotation to form the bulk copper/moly concentrate is still present in the concentrate after treatment, and readsorption of the collector may occur. In addition, some chemicals used as copper-sulfide depressants oxidize and lose their effectiveness over time. Another problem with chemical treatments, is the required handling of large amounts of reagents which are unsafe, toxic, and harmful to the environment. For alkali sulfides, as much as 50 pounds per ton of concentrate feed can be required. For cyanides (including ferro- and ferri-cyanides), up to 2 pounds per ton of concentrate feed are typically required.
The problem of readsorption of the collector can be largely eliminated by use of certain chemical oxidants which alters the copper-sulfide surface by destroying sulfhydryl collectors. Such a process is disclosed in U.S. Pat. No. 3,811,569 to Shirley et al. Therefore, the problem of readsorption of the collector is mostly eliminated. However, the safety, toxicity, and environmental problems persist.
The efficiency of some copper-sulfide depressants can be increased by thermal processes, such as steaming and roasting, which destroy or alter the previously added copper-sulfide collector, and change the surface of the copper and iron sulfide mineral particles. However, this efficiency is at the cost of extra process steps requiring significantly more process equipment, and increased energy costs.
Notwithstanding the measures in the prior-art processes to increase the efficiency of molybdenite recovery, the prior-art processes are inefficient. This is not only due to problems in selectively depressing the copper sulfides, but other factors contribute to the difficulty in recovering the molybdenite. For example, the type of molybdenite mineralization can contribute to the difficulty in recovery of molybdenite. Well-crystallized vein molybdenite does not cause serious problems in achieving a satisfactory recovery, but many porphyry ores contain molybdenite finely dispersed in quartz veins and molybdenite occurring as a film on other mineral phases, which can render the recovery of the molybdenite difficult. In addition, the presence of naturally floating impurities in the ore, such as talc and pyrophillite, also contribute to inefficiency in molybdenite recovery. Thus, the recovery of molybdenum from molybdenite/copper sulfide concentrates is limited, and for a commercially viable process, numerous conditioning steps and flotation stages are usually required to adequately separate and concentrate the molybdenite. Consequently, even minor improvement in molybdenum recovery would be desirable.
Ozone has been used in the art to treat sulfide ores. For example, Ishii, et al., Japanese Patent 70 16,322 (Chemical Abstract 101198h) "Flotation of Sulfide Ores," discloses the treatment of materials containing copper, lead and pyrite minerals with hydrogen peroxide or ozone oxidant. After treatment with the oxidant, the copper and lead minerals are separated from impurities such as pyrite, by floating both the copper and lead minerals into the froth product.
Natarajan and Iwasaki, in "Decomposition of Xanthane Collectors With Ozone in Alkaline Solutions," Minerals and Metallurgical Processing, November 1983, and Iwasaki and Malicsi in "Use of Ozone in the Differential Flotation of Bulk Copper-Nickel Sulfide Concentrates," Minerals and Metallurgical Processing, February 1985, disclose the use of ozone to remove residual xanthates in alkaline solutions from sulfide mineral surfaces, which enables the differential flotation of copper/nickel sulfide concentrates.
In the above references, the residual collectors are destroyed and additional collectors must subsequently be added to effect flotation.